Many tellurium containing metal oxide catalysts have been known. For example, it is known that catalysts composed of oxides of molybdenum and tellurium described in U.S. Pat. No. 3,164,626, catalysts composed of oxides of molybdenum, zinc and tellurium described in Japanese Patent Publication No. 7774/66, catalysts composed of oxides of tellurium and cerium described in U.S. Pat. No. 3,446,834, catalysts composed of oxides of molybdenum, tellurium, manganese and phosphorus described in U.S. Pat. No. 3,335,169, catalysts composed of oxides of iron, antimony, vanadium, molybdenum, tungsten and tellurium described in U.S. Pat. No. 3,668,147, catalysts composed of oxides of molybdenum, tellurium, antimony, cobalt and phosphorus described in Japanese Patent Application (OPI) No. 141724/79 (the term "OPI" as used herein refers to a published unexamined Japanese patent application), catalysts composed of oxides of tellurium, molybdenum and tungsten, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, tin or bismuth, etc. described in Japanese Patent Publication No. 16971/80, and catalysts composed of oxides of tin, antimony, copper, iron and tellurium, etc. described in British Pat. No. 1,595,008 are useful for an oxidation, ammoxidation or oxidative dehydrogenation reaction of organic compounds. For example, acrolein (or methacrolein) is formed by an oxidation reaction of propylene (or isobutene) and acrylonitrile (or methacrylonitrile) is formed by an ammoxidation reaction. Further, butadiene is formed by an oxidative dehydrogenation reaction of butene-1 or butene-2.
In the oxidation, ammoxidation or oxidative dehydrogenation reaction of organic compounds, the activity of the catalyst is often observed to deteriorate after use for a long period of time, though the degree thereof varies with the kind of the catalyst or the condition for using it.
Such deterioration of the activity arises from various causes, and a counterplan has been studied from various angles.
Such a phenomenon sometimes occurs in tellurium containing metal oxide catalysts, and the tellurium content in the catalysts sometimes is reduced together with a deterioration of the activity. It is supposed that the catalyst is subjected to irreversible reduction during the reaction and, consequently, tellurium is lost as elemental tellurium, organic tellurium compounds or tellurium hydroxides, etc. which have a comparatively high vapour pressure. However, the cause is not always obvious, because there are many cases that the deterioration of activity is not directly related to the decrease in tellurium content.
Whether the cause is obvious or not, it is important from the viewpoint of practical use to develop catalysts where deterioration occurs with difficulty, to establish a process by which the catalyst is deteriorated with difficulty, and to regenerate the deteriorated catalysts.
Although various processes have been proposed as processes for regenerating deteriorated catalysts, with all of them the processing is carried out after the catalyst is taken out of the reactor. Examples of them include a process for regenerating tellurium containing antimony oxide catalysts described in U.S. Pat. No. 4,049,575, a process for regenerating tellurium containing iron-antimony oxide catalyst described in U.S. Pat. No. 4,208,303 and a process for regenerating tellurium containing antimony compound oxide catalysts described in Japanese Patent Application No. 67872/80 (EPC Patent Application No. 0040950), etc.
In regenerating the deteriorated catalyst by these processes, the catalyst must be taken out after the reaction is stopped and, consequently, this causes a large economical loss due to production interruptions.
If the catalytic activity of the catalyst can be restored while the reaction is being carried out or without taking the catalyst out of the reactor even if the reaction is stopped, it would be very advantageous.
As to this, U.S. Pat. No. 3,882,159 discloses a process for regenerating catalysts which comprises contacting in situ a molybdenum containing fluidized-bed catalyst which has become deteriorated during an ammoxidation reaction of propylene with fluidized-bed particles composed of a substantially inactive carrier and molybdenum oxide at the reaction temperature. This U.S. patent is concerned with only regeneration of a molybdenum containing fluidized-bed catalyst, wherein molybdenum oxide supported on an inert carrier is simply used as the regenerating agent. The above described U.S. patent does not disclose a process for improving the activity of tellurium containing metal oxide catalysts.
U.S. Pat. No. 3,236,782 discloses a process for regenerating metal oxide catalysts containing at least Cr, V, Mo or W which comprises contacting the catalyst with a vapour of a compound of the same metal as that present in the catalyst. The process disclosed in this U.S. patent requires a complicated operation in that the catalyst component is introduced as a vapour into the reaction system. Further, this U.S. patent does not disclose a process for improving the activity of tellurium containing metal oxide catalysts.